Dexter,
If the caps have been sitting unused for an period of time, there should be no voltage on them at all. If there is a voltage, it has been put there comparatively recently. If the voltage that is showing is reversed in polarity, then someone hooked up the cap backwards.
Note that reversed polarity is not good for an electrolytic cap. (Age isn't either.)
When an electrolytic cap fails, it can explode, or it can vent through the little rubber plug found on the end of computer-grade electrolytics. (Smaller caps have a pre-stressed place in the can.) Neither event is something you want to have in your house, so I'd recommend charging the caps outside, especially if they've been subjected to a reverse charge. You might also want to wear some sort of eye/face protection. Incidentally, the only time I've ever had a cap vent on me, it smelled like a thousand dead cats, and was the very devil to get rid of. Explosions, I should hope, are fairly obviously not the sort of thing you'd want to have in your immediate vicinity.
As far as charging them slowly, the preferred way is to use a Variac. You might consider buying one, as they're pretty useful critters to have around. But in the meantime, a high resistance will slow the charging down to as slow as you want it; simply use a very large resistance in series with the cap.

I have used the resistor method and has worked at times better than a variac. one time i had a large electro Plow the Plug out at 1/10 its rated voltage. the resistor method will limit the Current and let the Voltage charge up to what ever level it can without doing any Strange stuff. the resistor should be a large value.

detxter,
I feel your pain, I tend to become emotionally attached to those big cans, I have an entire collection of them, and I have been more or less successful in returning them to life, 30 years may be a little too much though! If the cap have been reverse-polarized you should gently discharge them (that is very weird by the way). I usually try to keep the rate of discharge at a max of ~1V/sec. I found that cycles of slow charge-slow discharge at low voltage work pretty well in lowering the leakage fast. In your case you could start with 20-30V for a few times and then move up in voltage. It may take an hour to get the leakage down. Though, if things don't change for the best quickly it's time to put those babies to sleep. Also you should verify that after the procedure and an overnight rest the leakage is not gone bad again.
No point in building a new amp and using poor quality caps.

Personally the method I have been using is to hook up several wall mart wall plug transformers in series to get the voltage I need. Usually a couple of volts over the rated voltage. Then with 5kohm resistor in series let the caps chage for a day or two. Resistor should be mounted over something not flamable in case it plays fuse. After the cap is chaged well for couple of days, unconnect it and check that it takes couple of days for the voltage to bleed down. If leakage is too high/fast, don't use that cap. Believe this very slow charge to a slight overratting and checking for slow discharge gives me good test of whether or not the cap is good enough to use. Has worked well so far.

Dexter,
I've used a simple solution to the 'having no variac' problem:

It basically is a series resistor in the mains circuit. The 'resistors' I use a 25 / 100 W light bulb (we run 230V in NL). You'll need a power cord and two wall sockets. Wire the wall-sockets in series.
Plug a fitting with the smallest light bulb in one socket and the device which is 'under construction' in the other.
Do some measurements. If ok use the bigger bulb until your satisfied nothing serious is wrong, then attach the device under construction directly to main.